1. Field of the Invention
The present invention relates to a control system for rectifier equipment for transmission of high voltage direct current, the system including a control pulse emitter for the valves of the regulator unit and a regulator for setting the control angle of the control pulse emitter, the regulator being provided on its output side with a signal limiter for limiting the control signal when the voltage of the rectifier unit is increased towards a smaller control angle.
2. The Prior Art
Upon starting such a rectifier unit, the rectifiers are normally deblocked at a relatively great control angle -- upwards of 90.degree. or more -- which can be made by setting a high lower limit for the control angle. When, thereafter, this limit is reduced, the regulator of the rectifier unit will decrease the control angle and thus increase the direct voltage until the desired current or power is obtained.
However, if the inverter situated at the other end of the DC transmission for some reason does not start working and thus does not become conducting, no direct current will grow in the rectifier unit, and therefore its regulators will continue to increase the direct voltage of the rectifier unit, which direct voltage may reach considerable values, especially as the "open", that is, interrupted line, will reflect the direct voltage. This may cause the transmission line to be exposed to dangerous overvoltages. This is most serious in the case of cable transmissions, where the smoothing reactor and the cable together form an oscillating circuit, since overvoltages are more serious for a cable than for an overhead line.
To prevent the risk of overvoltages, a relatively high lower limit for the control angle of the rectifiers can be maintained until it has been established that the transmission really will be conducting, or the magnitude of the control signal, or the change of this magnitude, may be limited. In this way major overvoltages may be avoided, but at the expense of the control speed. A compromise solution which keeps overvoltages as well as the control speed within the desired limit seems difficult to reach.